scala3-compiler-bootstrapped/dotty.tools.dotc.transform.init/Semantic

Semantic

class Semantic

Companion: object

class Object

trait Matchable

class Any

Type members

Classlikes

Utility definition used for better error-reporting of argument errors

An object with unknown initialization status

Interpreter configuration

The (abstract) interpreter can be seen as a push-down automaton that transits between the configurations where the stack is the implicit call stack of the meta-language.

It's important that the configuration is finite for the analysis to terminate.

For soundness, we need to compute fixed point of the cache, which maps configuration to evaluation result.

Thanks to heap monotonicity, heap is not part of the configuration. Which also avoid computing fix-point on the cache, as the cache is immutable.

The environment for method parameters

For performance and usability, we restrict parameters to be either Cold or Hot.

Despite that we have environment for evaluating expressions in secondary constructors, we don't need to put environment as the cache key. The reason is that constructor parameters are determined by the value of this --- it suffices to make the value of this as part of the cache key.

This crucially depends on the fact that in the initialization process there can be exactly one call to a specific constructor for a given receiver. However, once we relax the design to allow non-hot values to methods and functions, we have to put the environment as part of the cache key. The reason is that given the same receiver, a method or function may be called with different arguments -- they are not decided by the receiver anymore.

A function value

Abstract heap stores abstract objects

As in the OOPSLA paper, the abstract heap is monotonistic.

A transitively initialized object

The abstract object which stores value about its fields and immediate outers.

Semantically it suffices to store the outer for klass. We cache other outers for performance reasons.

Note: Object is NOT a value.

A value which represents a set of addresses

It comes from if expressions.

Result of abstract interpretation

A reference to the object under initialization pointed by this

Abstract values

           Cold
  ┌──────►  ▲   ◄──┐  ◄────┐
  │         │      │       │
  │         │      │       │

ThisRef(C) │ │ │ ▲ │ │ │ │ Warm(D) Fun RefSet │ ▲ ▲ ▲ │ │ │ │ Warm(C) │ │ │ ▲ │ │ │ │ │ │ │ └─────────┴──────┴───────┘ Hot

The most important ordering is the following:

 Hot ⊑ Warm(C) ⊑ ThisRef(C) ⊑ Cold

The diagram above does not reflect relationship between RefSet and other values. RefSet represents a set of values which could be ThisRef, Warm or Fun. The following ordering applies for RefSet:

   R_a ⊑ R_b if R_a ⊆ R_b

   V ⊑ R if V ∈ R

An object with all fields initialized but reaches objects under initialization

We need to restrict nesting levels of outer to finitize the domain.

Types

Cache used to terminate the analysis

A finitary configuration is not enough for the analysis to terminate. We need to use cache to let the interpreter "know" that it can terminate.

For performance reasons we use curried key.

Note: It's tempting to use location of trees as key. That should be avoided as a template may have the same location as its single statement body. Macros may also create incorrect locations.

The state that threads through the interpreter

Value members

Concrete methods

Handles the evaluation of different expressions

Note: Recursive call should go to eval instead of cases.

Handle semantics of leaf nodes

Check that path in path-dependent types are initialized

This is intended to avoid type soundness issues in Dotty.

Evaluate an expression with the given value for this in a given class klass

Note that klass might be a super class of the object referred by thisV. The parameter klass is needed for this resolution. Consider the following code:

class A { A.this class B extends A { A.this } }

As can be seen above, the meaning of the expression A.this depends on where it is located.

This method only handles cache logic and delegates the work to cases.

Evaluate a list of expressions

Evaluate arguments of methods

Initialize part of an abstract object in klass of the inheritance chain

Compute the outer value that correspond to tref.prefix

Resolve C.this that appear in klass

Concrete fields

Extensions

extension (a: Value)

Conservatively approximate the value with Cold or Hot

extension (addr: Addr)

Ensure the corresponding object exists in the heap

Whether the object is fully assigned

It means all fields and outers are set. For performance, we don't check outers here, because Scala semantics ensure that they are always set before any user code in the constructor.

Note that isFullyFilled = true does not mean we can use the object freely, as its fields or outers may still reach uninitialized objects.

extension (thisRef: ThisRef)

extension (value: Value)

Promotion of values to hot

extension (value: Value)

Handle a new expression new p.C where p is abstracted by value

extension (value: Addr)

Can the method call on value be ignored?

Note: assume overriding resolution has been performed.

extension (values: Seq[Value])

extension (warm: Warm)

Try early promotion of warm objects

Promotion is expensive and should only be performed for small classes.

for each concrete method m of the warm object: call the method and promote the result
for each concrete field f of the warm object: promote the field value

If the object contains nested classes as members, the checker simply reports a warning to avoid expensive checks.

TODO: we need to revisit whether this is needed once we make the system more flexible in other dimentions: e.g. leak to methods or constructors, or use ownership for creating cold data structures.